DCS/ED 3-DOF starting project, need your help !

Many of your questions go to design choices rather than definitive answers, but perhaps my responses will encourage others to add their views.

1-I would like to start directly with 3DOF project, which is best to choose ? Heave or yaw for the last dof ?

That is really a question of design objectives, heave via a traditional 3DOF design would be better for planes and yaw would be better for choppers.

2-I read on this forum that for VR, the most important is speed and not angle. True ?

That is certainly my personal opinion.

3-what is the most immersive ? G-seat or motion simulator ?

Both have their own benefits, a G-seat can give some sense of sustained Gs while a motion simulator has more effect on the vestibule system. Their use is not mutually exclusive and they can be used together such as @Avenga76's rig and @SeatTime's planned upgrade to his 6DOF rig.

4-A lot of people use 24V DC motors. I work in hydroelectric factory and we have a lot of industrial AC motors. Could I use them ? is it possible or we can't control them ?

AC can work fine and would be a good choice for a high powered 3DOF rig for flight simulation, and many commercial 3DOF are AC powered: https://www.xsimulator.net/community/faq/vfds-and-3phase-motors.9/category

5-If I use 12V motors, could I use a 1250W ATX power supply to power them ?

Personally I think server or 12V LED PSUs are a much more cost effective option.

6-How do you manage to cancel simulator movements in Oculus Rift tracker ?

VectionVR offered a modified LibOVR motion cancellation file for free, which could be compiled with games to provide motion cancelation capabilities, but only 2 ever did. So the practical answer is to mount the Oculus camera on the rig, which works pretty well for flight sims, and some members here are currently experimenting with gyro stabilized gimbal mounts. You can have the Oculus camera off the rig for small axis movement sims when used for fast action, like race simulation, as you won't really notice the need for motion cancellation.

There are others but @speedy is a great help when it comes to AC use.

I am not sure I fully understand your question. Maximum torque is delivered at 90° and minimum at 0°.

Mechanical loss is related to the type of gears used and the gear ratio: http://www.meadinfo.org/2008/11/gear-efficiency-spur-helical-bevel-worm.html

There are different degrees of loss as I said, depending on ratio.

That said even taking mechanical loss into account powerful DC motors with worm gearboxes are incredibly good value for money as they can produce better potential performance than say SCN linear actuators at a fraction of the cost.

Do a basic sketch of the 3dof design you would like to build, post it and ask for members to comment.

There is a thread on cockpit design dimensions that may be of use: https://www.xsimulator.net/community/threads/sharing-cockpits-dimensions.6789/#post-76077

On my compact spring assisted 3DOF I have 2 motors at the front and 1 at the rear. I suspect it is about as small as it is practical to go, as the base is 500x500mm and the distance between the 2 front levers is only 535mmm and the rear lever is only 45mm out from the base frame. My HOTAS is mounted outside the connecting rods that go to the motors.

But the wider the footprint the more leverage, so going bigger than mine is a good idea.

In terms of your drawing I would suggest turning the motors around 180 degrees and indicate where you plan to connect levers.

@cthiggin is just finishing off his 3DOF, which uses 1hp AC Invertor Motors and heavy duty Boston Gearheads - 30:1 ratio: https://www.xsimulator.net/community/threads/questions-help-3dof-ac-platform-it-begins.5141/

Originally it was built with supported heave but that was not found to be necessary with the motor and gearbox combination, so I would think something of similar specs will work fine in practice.

While it is true more members use DC those who do use AC, like @speedy, are very knowledgeable and provide good support for others.

That is a traditional 3DOF design.

1. The longer the lever the greater the linear speed, but at the cost of torque, as that is reduced: https://www.xsimulator.net/community/faq/calculating-basic-linear-speed-and-forces.89/

2. The general community wisdom is that good motion speed is 150-700mm/s: https://www.xsimulator.net/community/faq/speed-needed-for-good-motion.218/

Speed for VR depends a bit on the sensitivity of the individual, what is being simulated (aircraft are usually slower motion than race sims), and the design axis movement distance. There is no precise way to define all those in advance, particularly personal sensitivity, so generally it would be a good idea to have some speed headroom, so personally I would aim for 250+mm/sec.

4. AC motors are powerful and I believe reliable, but @speedy is better qualified to comment on how best to ensure their longevity when used for motion.